1. Field of the Invention
The present invention relates to electron devices, and more particularly, to a switch tube adapted to rapidly change states between a high voltage non-conductive state and a high current conductive state.
2. Description of Related Art
High power switching devices are known in the art for switching between conductive and non-conductive states to provide short duration, high current pulses. A switching device must be capable of standing-off high voltages when in the non-conductive state, and rapidly switching to the high current conductive state with minimal voltage drop across the device. The high current pulses provided by a switching device have various applications in the art, such as plasma ion implantation, microwave tube current or voltage regulation, and the like.
Presently, there are two types of high power switching devices in common usage, and a third type disclosed in a previous patent by the inventor which has certain advantages over the two other types. The first type is the beam power tetrode switch tube generally comprised of a thoriated tungsten cathode wound into a cylindrical shape, a cylindrical control grid surrounding this, a screen grid, and finally a cylindrical anode outside the cylindrical screen grid. Usually, the control grid is run at an electric potential always negative with respect to cathode (if possible) to prevent interception of electrons on it and subsequent overheating. The control grid voltage is switched from a relatively high negative voltage in the beam off mode to a less negative voltage to switch the beam on. The screen grid is arranged to be in alignment with the control grid to shield it from electron interception. It is held at a potential that is positive with respect to cathode. Finally, the anode potential must be positive with respect to the cathode in order to receive electrons emitted from the cathode. There are many drawbacks to this first type of tube, including mechanical fragility of the wires comprising the cathode and grids, very high required cathode heater power, difficulty in alignment of the grid wires which can lead to grid interception and either grid emission or grid burnout, and other cathode, thermal and mechanical issues which affect reliability and which can lead to life problems when these tubes are used in high power applications.
The second type of switch tube in common use is the magnetron injection gun (MIG) type. This tube comprises a cylindrical cathode disposed concentrically within a modulating anode structure with a space defined between the cathode and the modulating anode. A Faraday cage collector is disposed axially from the cathode and modulating anode to receive the cathode current while preventing secondary electron emission. An axial magnetic field provided by an externally disposed electromagnet has flux lines that extend through the space into the opening of the collector. To switch the MIG switch tube to the conductive state, an electric potential, positive with respect to the cathode, is applied to the modulating anode causing current to be emitted from the cathode. The axial magnetic field bends the beam, preventing it from reaching the modulating anode, and directing it into the collector. While this type of switch tube has proven to be very reliable and long-lived, it has a generally higher voltage drop between the cathode and collector than other types of switch tubes making it less electrically efficient. Further, it requires an electromagnet and corresponding electromagnet power supply, which adds weight, complexity, and cost to the device.
The third type of switch tube comprises a shadow gridded tetrode device constructed from a plurality of electron guns, each having a cathode and an anode. A series of aligned grids is disposed between each cathode and anode, including a shadow grid closest to the cathode, followed thereafter by a control grid, and a screen grid. The tube also includes a suppressor grid following the screen grid having an opening generally equal to that of the edge of the cathode. In this tube, the anode includes cavities that provide a set of Faraday cage collectors to receive the cathode current. In operation, the tetrode switch tube is switched between the conductive and non-conductive states by controlling the voltage potential applied to the control grid. An example of this type of switch tube is provided by U.S. Pat. No. 4,745,324 to True, for HIGH POWER SWITCH TUBE WITH FARADAY CAGE ANODE. While the shadow gridded tetrode switch tube overcomes major limitations of both beam power tetrode and MIG switch tubes, it possesses a degree of complexity that makes it more expensive than standard beam power tetrodes, and less reliable than MIG switch tubes.
Accordingly, it would be desirable to provide a switching device having a high degree of current regulation with the ability to switch high current levels, fast switch response time, high voltage standoff capability, high switch efficiency, and very high device reliability, while overcoming these and other drawbacks of the prior art devices.